Review how to at:
https://developers.facebook.com/tools/debug/sharing/
https://developers.facebook.com/docs/sharing/
The meta tag og:image is used.
If you do not set them with a Social Sharing Plugin or even Yoast SEO (properly), then Open Graph guesses and pics one within the parameters.
The image to be shared must be a good quality image of minimum dimensions.
That said, it may be due to the DOCTYPE declaration:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "https://www.w3.org/TR/REC-html40/loose.dtd">
The scraper is not seeing all content and there are markup type errors.
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